UPPER MARLBORO, Md. — Shortly after Vice President Kamala Harris handed over the podium to President Biden on Thursday, dozens of attendees at a joint event simply stood up and left rather than listen to the retiring commander-in-chief.

The rude demonstration was a stark demonstration of the enthusiasm gap among Democrats revved up by Harris’ sudden elevation as the party’s presidential nominee — after Biden regularly suffered low rally attendance replete with heckling.

Hundreds of guests showed up to the event at Prince George’s Community College — standing in long, sweltering lines outside in bright sun and near-90-degree heat.

The event ostensibly was to celebrate the announcement of 10 prescription drug prices that will be lowered for senior citizens enrolled in Medicare Part D — with those savings taking effect in 2026.

Harris served as cheerleader for Biden at the event, leading the crowd there to see her in chanting, “Thank you Joe!” when referencing the reform, which was brought about by the Inflation Reduction Act of 2022.

Harris cast the tie-breaking vote in the Senate to pass that law, which also committed $369 billion to environmental projects, including grants to renewable energy companies and tax breaks for consumers, $64 billion to extend more generous Obamacare health insurance subsidies and $4 billion to Western wildfire resilience.

Biden dropped his re-election bid on July 21 and endorsed Harris — following a Democratic mutiny over concerns about his mental acuity following his disastrous June 27 debate performance against former President Donald Trump.

The sitting president appeared confused at that debate — claiming at one point he “finally beat Medicare.”

Biden seemed to recycle that badly botched line in his Thursday remarks, saying, “We finally beat Big Pharma.”

This is a developing story. Please check back for more updates.